SQL> show version;
ISQL Version: WI-V4.0.3.2937 Firebird 4.0
Server version:
Firebird/Windows/AMD/Intel/x64 (access method), version "WI-V4.0.3.2937 Firebird 4.0"
Firebird/Windows/AMD/Intel/x64 (remote server), version "WI-V4.0.3.2937 Firebird 4.0/tcp (HOME4)/P17:C"
Firebird/Windows/AMD/Intel/x64 (remote interface), version "WI-V4.0.3.2937 Firebird 4.0/tcp (HOME4)/P17:C"
on disk structure version 13.0
SQL> select cast('123'||x'00'||'321' as integer) from rdb$database;

        CAST
============
Statement failed, SQLSTATE = 22018
conversion error from string "123"
SQL> select cast('123'||x'00'||'321' as DECFLOAT(34)) from rdb$database;

                                      CAST
==========================================
                                       123